<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 300px;/*默认高度为内容高度*/
            background-color: rgba(0,0,0,0.3);
            padding: 10px;
            position: relative;
        }
        input{
            width: 260px;
            border: 0;/*去掉边框*/
            padding: 10px 20px;
        }
        img{
           position: absolute;
            top: 14px;
            right: 30px;
        }
        p{
            color: red;
            font-size: 12px;
            margin: 0;
        }
    </style>
</head>
<body>
<div>
    <input type="text" placeholder="请输入用户名">
    <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
    <p>用户名不能为空！</p>
</div>
</body>
</html>